Conventional rectifier circuits use either bipolar or Schottky diodes, both of which suffer from deficiencies.
Firstly, bipolar diodes have a higher forward voltage drop than Schottky diodes, so tending to introduce unacceptable power losses in high current circuits. In addition, reverse recovery currents can be large if the diode is switched from full conduction to blocking in a low impedance circuit. These currents can increase circuit stresses and power losses, as well as contributing to EMI (electromagnetic interference) from the host equipment.
Secondly, Schottky diodes, while possessing the advantage of lower forward voltage drops than comparable bipolar types, tend to show a more rapid rise of leakage current with temperature. This can lead to excessive power dissipation if reverse voltages are high.
It has been recognised that a power MOSFET (metal oxide semiconductor field effect transistor) possesses good characteristics for a rectifier. The forward voltage drop in `gate-on` mode can be very low and there is very little reverse recovery current at low frequencies of operation if the drain/body diode of the MOSFET does not carry the forward current. Also, high leakage currents do not occur and most MOSFETs can avalanche safely when subjected to overvoltage transients.
A number of circuits exist in which MOSFETs are used as active rectifiers, but these suffer from problems of control and timing. For example, if the active rectifier drive in a switched mode power supply is taken from the overall control circuit, which is often on the primary side of an isolation transformer, some means must be found to drive the active rectifier on the secondary side. This might involve the use of a second transformer. Because of different leakage inductance effects in the two transformers, this arrangement can give rise to imperfect timing between the main transformer output and the active rectifier drive. This causes power losses.
An alternative is to take the active rectifier drive from an overwind on the main transformer. However, this complicates the design of the transformer and is still not as good as using the actual secondary output since leakage inductance can again affect timing.
Other solutions offer secondary-side control and are based on switching the MOSFET by means of a comparator circuit. An example of a comparator based rectifier is given on page 237 of Electronics World for March 1999. Although the circuit of the present invention has some similarities to this approach, the design differs at least in that the control circuit and MOSFET operate in the linear mode.
This major difference reduces the problems caused by operating devices in the saturated mode, which saturation increases the time taken to change from one state to the other. The result of this time increase in switching the MOSFET OFF gives rise to an effect similar to reverse recovery current in a conventional diode.